Evaporator for concentrating radioactive solutions



June 1967 P. AUCHAPT ETAL EVAPORATOR FOR CONCENTRATING RADIOACTIVESOLUTIONS a Sheets-Sheet 1 Filed Sept. 14, 1964 mhqmhzwuzou a:

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INVENTORS 7 /5225 Hum/1 PT GEORQES Bouzou BY ROGER Seam/1r.

' ATTORNEYS J n 27, 967 P. AUCHAPT ETAL 3,327,763

EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS Filed Sept. 14, 19643 Sheets-Sheet 2 u: uJ '52 z m a a ,5 33 g 3% Tl CONCENTRATE FIG. 2

VAPOR AND uoum DROPLETS EQUALIZING PRESSURE LINE 248 ATTORNEYS 3Sheets-Sheet 3 June 27, 1967 P. AUCHAPT ETA-L EVAPORATOR FORCONCENTRATING RADIOACTIVE SOLUTIONS Filed Sept. 14. 1964 United StatesPatent 3,327,763 EVAPORATOR FOR CONCENTRATING ACTIVE SOLUTIONS PierreAuchapt, Georges Bouzou, and Roger Sautray, 1 Bagnols-sur-Ceze, France,assignors to Commissariat a IEnergie Atomique, Paris, France Filed Sept.14, 1964, Ser. No. 396,026 Claims priority, application France, Sept.19, 1963,

948,082; Jan. 30, 1964, 962,029

7 Claims; (Cl. 159-14) The present invention is concerned withanevaporator for concentrating radioactive solutions, as applicableespecially ,to the concentration of solutions of plutonium.

It is frequently necessary to concentrate radioactive solutions in orderto obtain a concentrated residual solution and a distillate which has alow level of activity. Thechief problem which arises from theconcentration of solutions of the kind referred to is the danger ofcriticality which, up to the present time, has imposed narrowlimitations on the maximum volume of solution which can be processed ina single oepration. v

The present invention is directed to an apparatus of relatively simpledesign which permits of rapid concentration of radioactive solutions andalso makes it possible to process a high flow rate and a high volume ofsolution, this being achieved without any attendant danger ofcrit-icality.

With this end in View, the evaporator according to the inventioncomprises a vessel which is intended to be partially filled with thesolution to be concentrated, 'atleast one thermo-siphon comprising alower branch which is connected to the base of said vessel, "a heatingbranch, an upper branch providing a return to the vessel and openingtangentially to said vessel, a pipe'for the evacuation'of the vaporizedsolvent which is connected tothe top portion of the vessel, a chamberfor supplying the solution to be concentrated anda chamber for thewithdrawal of the concentrated solution.

The vessel which is intended to be filled withthe solution to beconcentrated has a fiat cylindrical shape which is separated into twoparts in the direction of its height by a perforated stainless steelplate on which is supported a layer of Raschig rings formed, forexample, of boron glass; the vessel is coupled to the thermo-siphon bymeans of an annular pipe, at least the-internal portion of which islined with a neutron-absorbing material. If necessary, the outer portioncan also consist of a neutron-absorbing RADIO- ma'terial.

In accordance with a particular form of embodiment, the heating brancheshave a lobed right section which is preferably three-lobed, each lobebeing occupied by heating'means which are disposed substantiallyalongthe axis of'the lobe and the other branches of which have a flattenedcross section. The term"thermo-siphon which is employedv in thisdescription must be considered in a particular sense in which thecirculation of liquid which is initated by heating this latter isessentially due to a process of boiling in a vertical branch of aheating circuit. In this latter case, when the apparatus is in theinoperative" condition, the level ofliquid 'at rest'may occupy only thebottom of the fiat, vessel or even be located at a level below the upperbranch which provides a return to the ves'sel. It is only when, theapparatus is in operation that, the volume of liquid having increased asa result of boiling, the vessel is thus partly filled with the liquid.

The invention also consists of other arrangements which can usefully beemployed in conjunction with those refered to above but which can alsobe employed independently thereof. Allof these arrangements will becomemore readily apparent by consideration of the following "ice descriptionof two forms of embodiment of the invention which are given solely byway of example without any limitation being implied. The descriptionrelates to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view, partly in section, of an evaporatoraccording to the invention.

FIG. 2 is a diagrammatic view in elevation, partly in section, ofanother evaporator in accordance with the invention.

FIG. 3 is a diagrammatic sectional view, taken along the line III-III ofFIG. 2, of one portion of the :device of FIG. 2 as represented on alarge scale.

The evaporator which is illustrated in FIG. 1 shows the apparatusconsisting of a vessel 2 which is intended to be partly filled with thesolution to beevaporated. The vessel 2 is fitted at the bottom with avertical connectorpipe 4 and at the top with a connector-pipe 6 for thedischarge of vapor one of the heating circuits 14 being provided with alower, branch 16 which is connected to the connectorpipe 4, with aheating riser 20 and an upper branch 22 providing a return to the vessel2 which opens tangentially to said vessel and substantially at onehalfthe height of this latter. I I

, The heating branch 20consists of a conduit which is of larger diameterthan that of the branches 16 and 22 and in which is mounted a heatingrod 28 consisting-of a sheath of silica, for example, in which arefitted electric heating resistors having a suitable power rating and supplied through lead-wires or conductors such as the conductor 30.Provisionis made between the'sheath 28 and and the wall of the branch 20for a radial clearance within which the solution to be concentrated-isintended to circulate. The heating is designed to establish thecirculationjof the liquid by a rising film effect, a mixture of liquidand vapor being circulated through the heating branch 20. A heatexchanger 42 is located next in sequence to the outle t pipe "6 ofvessel 2 and performs the function of a condenser. Said heat exchangeris provided with an outlet 44 for draining-01f the condensate.-

The vessel 2 consists o'fa cylindrical portion 50 which is surmounted bya portion 52 having the shape or a spherical cap, the entire assemblybeing fabricated of stainless steel. Slightly below the top portion ofthe cylinder 50 and approximately at the height of the general level 54of the solution,- p rovision is made fora plate 56 of perforatedstainless" steel which supports a layer" of glass Raschig rings asrepresented diagrammatically at58, and the object of which is to preventthe entrainment of drops of liquid in the vapor phase as well as toforestall any dangerof c-riticality in the exceptional'event that thelevel of liquid were to'rise within the vessel. The top portion '52 ofthe vesselis fitted, apart from the'connectorpipe 6, with two additionalconnector-pipes, namely a pipe 60 whose function will bedefinedhere'inatter and a pipe 62 which is fitted witha' cock 64 andserves for decontamination. v

The outlet pipe 6 is coupled by means of an elbow 66 to a cyclone '68 ofknown type which, as a result'of the action of centrifugal force, isdesigned to retain'the drop lets of liquid which might otherwise havebeen carried away by the vapor phase. Owing toavery good vapordecontamination factor, this feature makes it possible to obtain adistillate which isas free as possible from radio active materials,andesp'ecially from plutonium. The but tom portion of the cyclone 68 iscoupled bymeans of an elbowed tube 70 to the conduit 72 which serves toconvey separated liquid towards the heating system. The top portion ofthe cyclone '68 is coupled to the condenser 42 of a known type which isreferred to as a coil'condenser in which a double coil of stainlesssteelis fed at a suitable rate with cooling water whichis admitted at 74and discharged at 76.

The admission and withdrawal of solution are carried out through theintermediary of two similar chambers 78 and 80 which each consist of aglass cylinder which is clamped between two metallic end-plates, the topendplate being designed to carry safety probes 82, 84 and 86, 88 forchecking the top and bottom levels in both chambers. The top portions ofsaid two chambers are connected by means of flexible pipes 90 and 92which form a T-junction so as to pass through a cooling unit 94 andterminate in the connector-pipe 60 of vessel 2, thus establishing anequalizing pressure relationship between the two chambers and the vesseland providing a first means of controlling the level of liquid in thevessel with a view to preventing abrupt variations in level which couldotherwise result from the decomposition into carbon dioxide gas ofcertain organic elements of the solution. The bottom portion of the feedchamber 78 is fitted with two pipes, namely a pipe 96 for pumping thesolution which is stored in the tank 98 by means of the pump 100 and a.pipe 102 for supplying the conduit 72 with solution. The withdrawalchamber 80 is also fitted with two pipes which operate as overflows,namely one pipe 104 which joins the bottom portion of the chamber 80 tothe conduit 72 and a pipe 106, the top end 108 of which determines thegeneral level 54 of the liquid and the bottom end of which terminates ina cooling unit 109 and in the outlet 110 1 through which the concentrateis delivered. Said chambers 78 and 80 are adjustable in height and makeit possible on the one hand to observe the general level of the liquidand, on the other hand, to provide a second means of regulating saidlevel according to the density of the concentrate.

In a preferred form of embodiment of the invention which is intended forthe concentration of plutonium oxalate mother-liquors and which will bedescribed hereinafter by way of example with reference to FIG. 1, thevessel 2 consists of a cylinder 50 of stainless steel 225 millimeters indiameter and 40 millimeters in height which provides for the liquid anevaporation surface of 398 cm?. Provision is made at 35 millimeters fromthe base of the separator for a stainless plate having a thickness of 3millimeters and perforated with holes 8 millimeters in diameter. Thesaid plate supports a layer -8 having a minimum thickness of '60millimeters and consisting of 10x 10 mm. Raschig rings, formed of glasscontaining 12.90% B 0 namely 4% boron. The condenser 42 comprises twocoils 12 0 of stainless steel having a total length of 13 meters andhaving a heat transfer surface area of 4,150 cm. and capable of removing10,500 kcal./h. by means of a flow of water of 925 l./h. (liters perhour), the temperature of the water at the inlet, at 74, being C. andthe temperature at the outlet, at 76, being 40 C.

The chambers 78 and 80 each consist of a glass tube 75 millimeters indiameter and 125 millimeters in height clamped between two metallicend-plates which carry the safety probes, the pressure Supply valves andthe liquid inlets and outlets The nest of heating tubes is entirelyconstructed of stainless steel and consists of three tubes such as thetube 20' which are parallel to each other and welded at theirextremities to two flat collectors 112 and 114 which provide a clearanceof 20 millimeters for the passage of the liquid. The immersed heaterelements such as the rod 28 which are located at a distance of at least40 centimeters from the vessel are formed of quartz and centered bymeans of six shoulder brackets which have not been shown in the drawingsand which are welded to the lower base of each tube such as the tube 20.Sealing joints of Teflon which are also not shown in the drawings arefitted at the top portion of the tubes 20 between these latter and theimmersion heater rods '28, the distance between the wall of the immersedheater rods and that of the tubes being 10 millimeters for the passageof the liquid. The three immersion heater elements have a unitary powerrating of 4 kw. at a maximum voltage of 220 v. Each immersion heaterelement is controlled in dependence on the safety probes 82, 84, 86 and88, mounted on the admission and delivery chambers 78 and is provided80, and with an alarm amrneter which shuts down the installation.

The heating power can be caused to vary by modifying the voltagedeveloped across the terminals of the heating resistors by means of the3-phase transformer, the rated power of which is 30 kvA. A manual-switchvoltmeter serves to determine the voltage developed across the terminalseach resistor.

Subject to possible heat losses which are in any case small and of theorder of 3% since the apparatus is heat-insulated, the volume ofdistilled liquid is proportional to the electric power supplied to theresistors. The table which is given below indicates the hourly volumedistilled as a function of the electric input power.

Power in kw.: Output of distillate in l./h. l2 15.5 11 14.3 10 13.0 911.7 8 10.4 7 9.1 6 7.8 5 6.5 4 5.2

A number of measurements of overall thermal efficiency have been takenand the results vary between 93 and 97%.

In an apparatus such as that which has just been described, the totalquantity of liquid contained is 4.4 liters.

The probes 82 and 84 of'the chamber 78 have for their object to correctany possible variations in delivery of the proportioning pump 100. Inthe event that the delivery of said pump decreases, the level dropswithin the chamber 78, the probe 84 is then uncovered, thereby producingthe stoppage of the heating and the lighting-up of an alarm signal. If,on the contrary, the delivery of the pump 100 increases, the level ofthe liquid rises within the chamber '78 and reaches the probe 82, thusproducing the stoppage of the installation and the lighting-up of analarm signal. A similar safety system operates within the chamber byvirtue of probes 86 and 88 in the event of crystallization occurringwithin the concentrate. Furthermore, if crystallization takes placewithin the evaporator vessel itself, the liquid no longer circulates,the evaporation process stops, and safety devices mounted on the outletof the cooling water of the condenser 42 and on the tubes 14 of theheating coil, initiate the total stoppage of the installation as well asthe actuation of an alarm signal.

FIG. 2 represents a second evaporator in accordance with the inventionwhich is intended to be placed within a hot cell separated by aleak-tight wall A into two compartments B and C. Compartment B containsthe evaporator proper and the devices for retaining those liquiddroplets which are entrained by the vaporized solvent and for returningthem to the evaporator. Compartment C contains the ancillary devicessuch as' the condenser for recovery of the solvent and devices foradmission of the solution to be concentrated and for draining-01f thesolution once it has been concentrated.

The evaporator comprises a vessel 202 formed of two assembled partswhich has a substantially cylindrical shape and a small height withrespect to its diameter. During operation, the effluent in liquid phasewhich is to be concentrated occupies the bottom of said vessel to adepth which must remain sufliciently small to prevent any criticalityhazard. The vessel 202 contains boron glass Raschig rings 236, saidrings being located above the efiluent and designed to retain anydroplets of solution which'could otherwise be carried away by thevaporized solvent.

The vessel 202 is fitted at the bottom portion thereof with a verticalannular tube 204 having a thickness which is small with respect to itsdiameter and is fitted at the top portion thereof with a connector-pipe206 for the withdrawal of the solvent in vapor phase. The extremity ofthe vertical tube 204 is fitted with a pipe 212 for the purpose offilling the vessel with effluent to be concentrated and for returning tothe solution the droplets which have been entrained by the vapor. Thesaid pipe 212 is fitted with a device which has not been illustrated butwhich is of similar design to the admission chamber 78 of FIG. 1 andserves to regulate the level of the liquid within the vessel 202.

' There are associated with the vessel 202 two heating circuits 2'14 and214', although this number is not given by way of limitation. Thesecircuits are of similar design and, consequently, only the circuit 214will be described. The circuit 214 comprises a lower branch 216 (asshown in FIGS. 2 and 3) having a common trunk 217 subdividing into twoconduits 218 (FIG. 3), two heating elements 220 and an upper branch 222which also comprises two lateral conduits 224 which return to the vessel202 via a common trunk. Each branch 220 is associated with a lateralconduit 218 and a lateral conduit 224.

The recovery of the solution once this latter has been concentrated iseffected by means of a conduit 226, said conduit being connected to adelivery chamber which has not been shown in the drawings but which issimilar to the chamber 80 of FIG. 1 and which is fitted with suctionmeans.

The heating branches 220 of the evaporator according to the inventionconsist of conduits having a lobed cross section. There is mounted ineach lobe an immersion heater rod 228 which is formed of silica, forexample, and which is fitted with electric heating resistors having asuitable power rating and supplied with current by means of leads 230..A radial clearance through which the solution to be concentrated isintended to circulate is formed between the immersed heater rod 228 andthe wall of the branch 220. When the immersed heater rods 228 aresupplied with current, a circulation of liquid by a rising film effectis established in the direction indicated by the arrows f. Thiscirculation can be accelerated by making provision for a pump, but thiscomplicates the system and is not usually desirable.

The use of a lobed cross section and not a cylindrical section permits asmaller volume for a given heating perimeter, which is an advantageousarrangement from the point of view of criticality hazards.

The lower and upper branches 216 and 222 have a flat rectangular crosssection, namely a height which is small with respect to the widththereof, the object of this arrangement being to rule out any danger ofcriticality.

The fluid-circuits 214 and 214' which have been shown in FIG. 2 eachopen tangentially into the vessel 202 in diametrically opposite zonesand in opposite directions.

In order to complete the separation of liquid phase and vapor phase asinitiated by the boron glass Raschig rings 236, the discharge pipe 206opens into a cyclone separator 234 which can be of any conventionaltype. The vaporized solvent flows in the direction shown by thebroken-line arrows in FIG. 2 and the vertical flow motion which isgenerated within the separator tends to remove liquid droplets.Accordingly, after being freed from the liquid droplets which mightstill have been contained therein, the vapor phase passes out of theseparator into a conduit 237 which traverses the partitionwall A andopens into a condenser 242. The droplets which are retained within thecyclone return to the conduit 212 through a pipe 238 which is fittedwith a fluid trap 240 (arrows F in FIG. 2). The condenser 242 consistsof a heat exchanger of conventional design through which cooling wateris circulated. The condensed solvent is withdrawn through thedraining-off pipe 244 containing valve 246.

The pressures which prevail within the admission chamber and deliverychamber (which have not been shown in the drawings) and within thevessel 202 are balanced by means of a conduit 248 (as shown in FIG. 2)which is similar to the conduit -92 of FIG. 1.

In order to prevent neutron interaction between the different componentunits of the evaporator, certain opposite facing walls of said differentcomponent units of the evaporator in accordance with the invention arecovered with a material which has a high neutron capture cross-section.It is possible, for example, to make use of sheets of cadmium foil whichare applied in close contact with the walls.

The arrangements which have just been described make it possible withoutdifiiculty to achieve subcriticality on the one hand of each componentunit and on the other hand of the evaporator as a whole while alsopermitting of small'overall size. It is of interest to note the following numerical values which are given by way of example and whichcorrespond to an apparatus for the continuous processing of solutions ofplutonium with an evaporation rate of the order of 60 l/h.

The characteristics of the apparatus are such that each unit satisfiesat least one of the following conditions as regards the solutioncontained therein:

Volume of vessel lit-ers Smaller than 4 Diameter of vessel mm Less thanDepth of liquid mm Less than 40 The vessel 202 consists of a cylindricalchamber 450 millimeters in diameter, the bottom of which is filled withsolution up to a maximum depth of 35 millimeters during operation andthe top portion of which is occupied by boron-type Raschig rings up to aheight of approximately 60 millimeters.

Each heating circuit comprises two heating branches having a three-lobedright section and each fitted with three immersion heater elementshaving a power rating of 4 kw. The lower and upper branches 216 and 222have a height of 20 millimeters and a width of millimeters whereas theconnector-pipe 204 has an external diameter of millimeters and a radialwidth of 20 millimeters.

Each unit therefore satisfies at least one of the conditions ofsubcriticality. In order to prevent any appreciable neutron interactionbetween the different units, it is merely necessary to provide aclearance of 400 millimeters between these latter; in the apparatusdescribed, the clearances provided are substantially in excess of thisvalue. The presence of neutron-absorbing cladding material such ascadmium and polythene reduces neutron interaction even further. In fact,the geometry of the entire assembly makes it possible to dispose blocksof polythene or any material of the same type between the units referredto in order to take part in the absorption both of neutrons and of gammaradiation which is induced within the cadmium by the neutrons. Suchblocks 250, 252 and 254 are represented diagrammatically in chain-dottedlines in FIGS. 2 and 3.

As will readily be apparent, the invention is not limited to the formsof embodiment which have been illustrated and described by way ofexample and it will be understood that the scope of this patent extendsto any equivalent arrangement.

What we claim is;

1. An evaporator for concentnating radioactive solutions comprising:

(a) a vertical vessel of substantially circular crosssection which isintended to be partly filled with the solution to be concentrated;

(b) a plurality of thermo-siphons each comprising:

(1) a lower solution conveying branch duct communic'ating with the baseof said vessel,

(2) an upright solution heating branch duct commnnicating with saidlower branch duct-consisting of a vertical pipe and at least onevertical heater rod concentrically disposed therein forming an annularspace for movement of said solution andvapor and (3) an uppersolution-conveying branch duct communicating said heating branch ductwith said vessel and opening tangentially into said vessel; (c)' a pipefor the evacuation of vaporized solvent which is connected to the topportion of the vessel; (d) a respective feed chamber communicating witheach thermo-siphon in feed-supplying relation there- (e); a withdrawalchamber communicating with each themo-siphon in concentrate-removalrelation thereto, and

(f) means in said withdrawal chamber for regulating the level of thesolution in said vessel to a substantially constant height.

2. An evaporator according to claim 1 in which the liquid containingportions are of subcritical dimensions.

3. An evaporator in accordance with claim 1 wherein the, pipe for the,evacuation of the vaporized solvent is provided with a vapor-liquidcyclone separator whose bottom portion is connected to and communicateswith the lower branch duct of the ther-mo-siphon by a return line forthe liquid and is provided with a condenser located downstream of saidcyclone separator on said evacuation pipe for condensing said vapor.

4. An evaporator in accordance with claim 3, wherein blocks of amaterial absorbing neutrons and gamma radiation are interposed betweenthe vessel and the heating branches.

5. An evaporator for concentrating radioactive solutions comprising:

(a) a vertical vessel of substantially circular crosssection and whichis intended to be partly filled with the solution to be concentrated;

(b) at least one thermo-siphon comprising:

(1) a lower branch duct connected to and communicating with the base ofsaid vessel,

(2) a solution heating branch duct communicatingwith said lower branchduct consisting of a vertical pipe having a lobed cross-section and.

one vertical heater rod disposed substantially concentrically in eachlobe forming an annular lobe-shaped, rod-surrounding space for upwardmovement of said solution and generated vapor and (3) an upper branchduct communicating with said heater branch duct and said vessel at theirupper ends providing a return to said vessel and openlng tangentiallyinto said vessel; (c) a pipe for the evacuation of vaporized solventconnected to the top portion of the vessel;

((1) means for regulating the level of solution in said.

vessel and communicating therewith; (e) means for supplying saidsolution to be concentrated to said vessel and communicating therewith;V

and (f) means communicating with a bottom portion of said vessel forwithdrawing concentrated solution.

References Cited UNITED STATES PATENTS 1,191,108 7/1916 Kestner 159l41,819,517 8/1931 Lichtenthaeler 122 -133 X 2,605,38i1 7/1952 Head2l9--284 X 3,181,593 5 /1965 Lindley Q l59-1 3,205,934 9/1965 Vincenteta1. l5947 NORMAN YUDKOFF, Primary Examiner.

I. SOFER, Assistant Examiner.

1. AN EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS COMPRISING: (A)A VERTICAL VESSEL OF SUBSTANTIALLY CIRCULAR CROSSSECTION WHICH ISINTENDED TO BE PARTLY FILLED WITH THE SOLUTION TO BE CONCENTRATED; (B) APLURALITY OF THERMO-SIPHONS EACH COMPRISING: (1) A LOWER SOLUTIONCONVEYING BRANCH DUCT COMMUNICATING WITH THE BASE OF SAID VESSEL, (2) ANUPRIGHT SOLUTION HEATING BRANCH DUCT COMMUNICATING WITH SAID LOWERBRANCH DUCT CONSISTING OF A VERTICAL PIPE AND AT LEAST ONE VERTICALHEATER ROD CONCENTRICALLY DISPOSED THEREIN FORMING AN ANNULAR SPACE FORMOVEMENT OF SAID SOLUTION AND VAPOR AND (3) AN UPPER SOLUTION-CONVEYINGBRANCH DUCT COMMUNICATING SAID HEATING BRANCH DUCT WITH SAID VESSEL ANDOPENING TANGENTIALLY INTO SAID VESSEL; (C) A PIPE FOR THE EVACUATION OFVAPORIZED SOLVENT WHICH IS CONNECTED TO THE TOP PORTION OF THE VESSEL;(D) A RESPECTIVE FEED CHAMBER COMMUNICATING WITH EACH THERMO-SIPHON INFEED-SUPPLYING RELATION THERETO; (E) A WITHDRAWAL CHAMBER COMMUNICATINGWITH EACH THERMO-SIPHON IN CONCENTRATE-REMOVAL RELATION THERETO, AND (F)MEANS IN SAID WITHDRAWAL CHAMBER FOR REGULATING THE LEVEL OF THESOLUTION IN SAID VESSEL TO A SUBSTANTIALLY CONSTANT HEIGHT.